This article describes the multicriteria Monitoring and Alert System designed to allow safe access to Santos Morcillo Lake (located in Ruidera Lakes Natural Park, Central Spain). In the spring of 2007, two families of cracks were observed on both banks of the lake, located near the barrage separating it from the downstream Batana Lake. A sinkhole was also detected. This raised great community alarm, since it was feared that the barrage would collapse. A geoelectric and geotechnical survey was carried out to monitor the crack aperture, the barrage movements and the evolution of the lake water level. On the basis of the information obtained, a model was designed that was able to offer a simple and consistent explanation of the system’s behaviour. Therefore, the model was used as a basis to define the safety assessment system proposed here. The system is based on the data from previously installed monitoring devices that would be processed daily by the Technical Staff of the Park. In the event that a warning should be triggered, an integrated analysis of all the information must be conducted, and an alert would be activated. The alert level will depend on how reliable the mobilisation hypothesis is, and different degrees of restricted access to Santos Morcillo, Batana and Colgada Lakes will take effect.